Recently, along with the performance improvement of an information processing apparatus, data rate has been increased for a data signal transmitted or received within an apparatus or between apparatuses.
A reception circuit of the data signal determines an amplitude level of the data signal at timing matching a sampling clock, and regenerates data according to the determination result. When the data rate is increased, slight phase shift generated between the data signal and the sampling clock affects the accuracy of the data determination. Accordingly, there has been utilized a technique called tracking CDR (Clock and Data Recovery) which detects such phase shift and synchronizes the phase of the sampling clock with the phase of the data signal.
In the reception circuit handling the data signal having a high data rate, there is frequently used a technique called 2× tracking CDR (hereinafter, described as 2×CDR) which performs sampling twice in an interval of 1 UI (Unit Interval). 2×CDR uses a clock to match an edge with the eye center of the data signal and a clock to match the edge with a transition point (boundary) at which the amplitude level of the data signal makes a transition. In the following, the former clock is called a data clock and the latter clock is called a boundary clock. Here, the both clocks are adjusted by a phase adjustment circuit so as to have a phase difference of 0.5 UI. Thereby, when the boundary clock follows the transition point in CDR, it is possible to perform sampling (data determination) at the eye center of the data signal using the data clock.
Japanese Laid-Open Patent Publication No. 2003-318872
Japanese Laid-Open Patent Publication No. 2002-232404
The data clock and the boundary clock are input into a determination unit (comparator) which performs sampling of the data signal. At this time, a delay difference (skew) is generated until the clocks arrive at the comparator because of the difference in transmission path and load between the data clock and the boundary clock. Therefore, even if the edge of the boundary clock follows the transition point of the data signal, there is a possibility that the edge of the data clock becomes unable to locate the eye center of the data signal and the data determination accuracy is deteriorated.